Diffusers are used to scatter light such that an object within the resultant diffuse light pattern is illuminated uniformly by light propagating from a substantially increased number of angular directions.
Conventional diffusers are typically made from a glass pane or a plastic sheet processed to have a light scattering refractive surface and/or impregnated with dispersed light scattering diffractive particles. A typical diffuser might be the light-transmissive shell of a frosted light bulb, or a ground glass plate or other rough surface facing a light source for the diffuse reflection of light originating therefrom. Conventional diffusers have been used in the display industry, for example, to define the quality (cf., angular distribution) and quantity (cf., brightness) of image light available to a viewer.
Important parameters considered in the design of a diffuser are the uniformity of the projection of light transmitted or reflected in two dimensions, the efficiency of transmission or reflection as a function of wavelength over the visible wavelength region, and the solid angle of view for which the diffuser is operational. Conventional diffusers are generally limited in their capacity to accomplish desirable results in any one of these parameters without some sacrifice to the others. Holographic diffusers--having good forward scattering properties--are not so limited, and are lighter, more compact, and can generally be formed and replicated with comparatively greater ease. A holographic diffuser can provide high uniformity, high forward efficiency (i.e., with minimum backscatter) and a controllable solid angle of view largely independent of incident angle.
The inherent and controllable properties of holographic diffusers has generated interest in their application to image display systems. For example, reflective holographic diffuser elements have been employed to effect the illumination of liquid crystal displays and like electrooptical displays. See e.g., PCT International Publication No. WO 95/12826, published May 11, 1995; U.S. Pat. No. 5,659,408, issued to M. M. Wenyon on Aug. 19, 1997; and U.S. Pat. No. 5,418,631, issued to J. M. Tedesco on May 23, 1995. Likewise, the use of transmissive holographic diffusers have been employed in place of traditional light diffusing lenticular optics in rear projection screen television systems. See, e.g., D. Meyerhofer, Holographic and Interferometric Viewing Screens, APPLIED OPTICS, Vol. 12, No. 9 (September 1973); U.S. Pat. No. 3,909,111, issued to D. Meyerhofer on Sep. 20, 1975; J. M. Tedesco et al., Holographic Diffusers for LCD Backlight and Projection Screens, SID 1993 DIGEST, Ch. 5.3, pp. 29-32 (1993); U.S. Pat. No. 5,365,354, issued to Jannson et al. on Nov. 15, 1994; and U.S. Pat. No. 5,534,386, issued to Petersen et al. on Jul. 9, 1996.
While the above references report advances in screen resolution, brightness, efficiency, and image scattering, they do not address or acknowledge or are concerned with the long-standing problem of lateral color shift--a problem especially pronounced in multi-"color tube" projector systems and still addressed for the most part with conventional non-holographic solutions. See U.S. Pat. No. 4,374,609, issued to H. G. Lange on Feb. 22, 1983; U.S. Pat. No. 4,573,764, issued to R. H. Bradley on Mar. 4, 1986; U.S. Pat. No. 4,762,393, issued to G. B. Gerritsen on Aug. 9, 1988; U.S. Pat. No. 4,919,515, issued to S. Hasegawa et al. on Apr. 24, 1990; and U.S. Pat. No. 5,066,099, issued to T. Yoshida et al. on Nov. 19, 1991. Unfortunately, conventional solutions--such as the use of a lenticular sheet having a vertically-striped pattern of light-absorbers and light-bending lenticules ("black stripe")--are accomplished with sacrifices to image resolution and brightness. The present invention provides a holographic solution.
Color projection systems generally employ three picture tubes, one for each of the primary colors. In the conventional arrangement only one centrally located picture tube is aligned with the normal axis of the projection screen, and the other two are laterally displaced therefrom. Since the scattering pattern effected by conventional diffusing optics is largely dependent on incident input angles, the angular variance of the offset tubes results ultimately in different mixes of signal strengths at different horizontal viewing locations. Because each tube projects a different color, these different signal strength mixes--without optical correction--will be perceived as different color balances. Thus, while a viewer in a central viewing zone may see a chromatically-balanced image, those in viewing zones to the left and right may see, for example, a red-dominant image or a blue-dominant image.
The existing difficulty in accomplishing good color balance in rear projection screen televisions through holographic means can be attributed to the impracticality of current holographic color correction strategies. The known strategies are not easily amenable to the manufacture of a hologram having the appropriate screen optics, i.e., a hologram capable of scattering diffuse light homogeneously throughout a relatively wide horizontal viewing zone. For example, in their attempt to provide a display having multiple discrete off-axis fields of view (see, U.S. Pat. No. 5,046,793, issued Sep. 10, 1991), Hockley and Pawluczyk suggest designing a holographic diffuser by a stepped methodology to provide chromatic correction within predetermined "eyeboxes". For each eyebox, several angle-specific holographic exposures are recorded that provide collectively the appropriate mix of signal strengths within said eyebox. While the Hockley and Pawluczyk method may be useful for designing heterogeneous displays having two or more discrete viewing zones, the methodology would require an impractical number of holographic exposures to define good diffusing properties homogeneously and in perceived continuum throughout a wide viewing angle.
In light of the above, need is present for a comparatively easy-to-manufacture holographic transmission diffuser, especially those capable of effecting homogenous lateral color constancy in rear projection screen displays.